The subphylum Chelicerata constitutes one of the major subdivisions of the phylum Arthropoda, including the arachnids, horseshoe crabs, and related forms. These mainly predatory arthropods ultimately outlasted the now extinct trilobites, the common marine arthropod of the Cambrian era. Most of the marine chelicerates, including all of the eurypterids, are now extinct. The chelicerates and their closest fossil relatives (mostly originally included in the Xiphosura) are grouped together with the trilobites to form the taxon Arachnomorpha.
The chelicerae, which give the group its name, are pointed appendages that grasp the food in place of the chewing mandibles most other arthropods have. Most are unable to ingest anything solid, so they drink blood or spit or inject digestive enzymes into their prey. The legs on the prosoma are either uniramous or have a very reduced gill branch, and are adapted for walking or swimming. The appendages on the opisthosoma, in contrast, are either absent or are reduced to their gill branch.
The Chelicerata are divided into five classes:
The Pycnogonida actually show some strong differences from the body plan as described above, and it has been suggested that they represent an independent line of arthropods. They may have diverged from the other chelicerates early on, or represent highly modified forms. Sometimes they are excluded from the Chelicerata but grouped with them as the Cheliceriformes. The name Merostomata should be avoided because in all recent cladistic hypotheses it refers to a paraphyletic group composed by the Xiphosura + Eurypterida.
The Burgess shale animal Sanctacaris, and perhaps the aglaspids, have also been suggested as belonging here, but this is now disputed. These are extinct forms that arose in the Cambrian; the aglaspids are believed to have died out during the Silurian. After The oldest group of unequivocal chelicerates are the Pycnogonida, found from the late Cambrian onwards.